1. Field of the Invention
The invention relates to a fault detection arrangement for electric consumers in motor vehicles. More particularly, the invention relates to a fault detection arrangement to detect the function and/or malfunction of an electric consumer in a motor vehicle.
2. Description of the Related Art
In electrical circuits, such as those provided in motor vehicles for operating illumination devices, for example light units, headlights and lamps, for operating actuators, for example electrically adjustable external rear-view mirrors, electric window winders, electric seat adjustment means and electric or electronic locking mechanisms, as well as for operating drive motors, for example of pumps, fans and compressors to name just a few conceivable variants, one or more electric consumers are arranged, the function or unheralded malfunction of which or the unexpected failure of which may be relevant to safety.
It is therefore desirable to monitor the function and/or malfunction and/or failure of electric consumers in the circuit, for example so as to deliver a warning, for example to a driver of a motor vehicle, if a malfunction is expected, if there is a fault that cannot be directly detected visually, or if function is limited. This type of monitoring is generally known as fault detection.
In this instance the fault detection arrangement also increasingly comprises, in addition to an operation monitoring system, an idle monitoring system that detects an expected function or malfunction or a fault whilst the consumer is not operated and is idle. In addition to short circuit and burn-out, it should also be taken increasingly into consideration that an electric consumer could fail not only as a result of its own breakdown, but also as a result of an interruption to the terminal connecting lines (also called terminals for short), this type of failure being referred to as open load. Open load therefore differs from an intentional interruption of the terminal, achieved by a switch, since the consumer is actively separated and disconnected from the power supply by a switch, whereas when the terminal is interrupted there is an unintentional and therefore unforeseeable malfunction of the consumer.
Fault detection of electric consumers is known from U.S. Pat. No. 7,280,333 B2. The fault detection arrangement first determines whether an ohmic consumer or the terminal thereof is operating normally or is defective. If a defective state is determined, the fault detection arrangement then ascertains whether it is a case of open load, a short circuit to ground or a short circuit to supply voltage or phase. The fault detection arrangement further provides for a parameter of the electric consumer to change if a defective state is determined, so as to evade the defective state. For example the direction and/or strength of a current flowing into or out of the ohmic consumer is/are changed. For this purpose the fault detection arrangement comprises a detection circuit that is connected to a measuring point. An electric consumer provided to monitor fault detection is connected to ground at the measuring point. The detection circuit has two comparators, a switch, two variable power sources and a connecting line to the measuring point. The two variable power sources are connected to the connecting line, one of the power sources being variable to ground and the other being variable to supply voltage. The switch is connected on one side to the connecting line. An input of a comparator is connected, in each case, on the other side of the switch in such a way that the two comparators are interconnected via an input and can be connected to the measuring point by the switch, optionally via the connecting line. The remaining free input of one comparator is connected to ground and the free input of the other comparator is connected to supply voltage. In order to determine whether an ohmic consumer or its terminal is operating normally or is defective, the switch is closed so the comparators are connected to the measuring point. The comparators therefore monitor whether the voltage of the measuring point lies below a lower threshold value or above an upper threshold value. The outputs are read, a fault-free, normal operating state being present if the outputs of both comparators are low or zero, and a defective state being present if one of the outputs is high or one. If a defective state is determined, it is ascertained whether it is a case of open load, a short circuit to ground or a short circuit to supply voltage or phase by a change to a predetermined direction and a predetermined strength of a current flowing into or out of the measuring point caused by the variable power sources. If the switch is closed the comparators therefore again monitor whether the voltage of the measuring point lies below the lower threshold value or above the upper threshold value. If the voltage lies below the lower threshold value the ohmic consumer is short-circuited to ground. If the voltage lies above the upper threshold value it is attempted to allow current to flow into the measuring point so as to allow the voltage of the measuring point to fall below the upper threshold value. If this works, it is a case of open load. If this fails to work and the voltage remains above the upper threshold value, the consumer is short-circuited to supply voltage.
A drawback of this is the high cost of both the detection circuit, with its many complex and expensive components, and the temporal and logical control of the components in the detection circuit and of the interrogation algorithms for carrying out and evaluating the two-stage fault detection process.
A fault detection system comprising a comparatively simple switch is known from DE 10 2004 045 435 B4. In this instance the current or power consumption of an electric consumer is monitored and, by comparison with an upper, a middle and a lower threshold value, it is decided whether a short circuit is present, a light bulb is operated as a consumer, a light diode is operated as a consumer or whether there is no current flowing and therefore the consumer is burnt-out. For this purpose the upper threshold value is set at 50 W, above which the presence of a short circuit is assumed. The middle threshold value is set at 10 W, light bulb operation being assumed with a power consumption between 10 W and 50 W. The lower threshold value is 0.1 W, light diode operation being assumed with a power consumption between 0.1 W and 10 W. A defect of the electric consumer is assumed below a power consumption of 0.1 W. The aim of fault detection is to connect an electric resistor during light diode operation, for example of a repeat indicator light of a motor vehicle, in such a way that, without the need for retrofitting measures or with any usability of light diodes or light bulbs from the perspective of a control device of the motor vehicle, a sufficiently high amount of power is taken up so as to assume normal light bulb operation and still detect a fault caused by short circuit or burn-out or malfunction.
A considerable drawback of this fault detection arrangement is the high level of power dissipation, for example until a short circuit is detected.
A fault detection arrangement is known from DE 10 2008 042 595 B3 and also uses light diodes as lighting means in existing motor vehicles, without having to carry out any retrofitting measures. However, in this instance power dissipation of conventional lighting means is not produced by a resistor, which simply burns the power not required, but power dissipation is instead simulated, the electric power not required to operate light diodes, for example, compared to light bulbs being withdrawn from the onboard supply of a motor vehicle at one point and fed back at another point.
Drawbacks of this fault detection system include the high levels of current flowing together with a correspondingly expensive dimensioning of the electric terminal connecting lines.
US20040004483 A1 discloses a circuit comprising two switches and a voltage divider arranged parallel to the two switches. Fault detection is achieved by comparing a plurality of potentials in a comparator.
US20040145838 A1 discloses a circuit in which a large number of potentials are tapped via a plurality of switches in order to detect engine failure.